The radio measurements performed by a user equipment (UE) can be absolute or relative. Absolute measurements are done on a single cell, or the measurements are not compared with some reference value (i.e., they are not relative to other values). An absolute measurement may also involve two cells, or the measurements done on signals of a cell are not compared with some reference value or reference measurement component. Typically, a relative measurement result is obtained by the UE by comparing the measurements done on any two cells. An example of a relative measurement is a relative signal strength comparing strengths measured on a serving cell and a neighbor cell. The relative measurement quantity can be expressed in linear scale or in log scale. In the former case, the relative measurement quantity is the difference between the two measurement components, and is expressed in dB. A reference value can be pre-defined or configured by a network node.
Currently, only relative timing measurements may be reported by a UE in LTE. More specifically, only reference signal time difference (RSTD) measurements may be reported by a UE in LTE. According to the 3GPP TS 36.214, the RSTD in LTE can be described as the relative timing difference between a neighbour cell j and a reference cell i. It is defined as TSubframeRxj−TSubframeRxi where: TSubframeRxj LS the time when the UE receives the start of one subframe from cell j, and TSubframeRxi is the time when the UE receives the corresponding start of one subframe from cell i that is closest in time to the subframe received from cell j. The reference point for the observed subframe time difference is the antenna connector of the UE. The RSTD is applicable for RRC_CONNECTED intra-frequency and RRC_CONNECTED interfrequency.
Accuracy requirements exist for absolute reference signal received power (RSRP) and absolute reference signal received quality (RSRQ) measurements, as well as for the relative intra-frequency RSRP and relative inter-frequency RSRQ measurements specified in TS 36.133. Currently, these relative measurements are used only for event triggering. An example is event A3, when a neighbor cell becomes offset better than the primary cell, where the cell(s) that triggers the event is on the frequency indicated in the associated measObject, which may be different from the (primary) frequency used by the primary cell. Another example is event A6, when a neighbor becomes offset better than the secondary cell, where the neighbor(s) is on the same frequency as the secondary cell. In other words, both are on the frequency indicated in the associated measObject, and it is therefore important to ensure that the triggering occurs with a sufficient degree of accuracy.
Such requirements for relative intra-frequency RSRP have also been specified for enhanced inter-cell interference coordination (eICIC) and further enhanced inter-cell interference coordination (FeICIC), where RSRQ measurements are restricted to certain subframes indicated by a measurement resource restriction pattern, and the patterns may be different for the primary cell and neighbor cells. RSRP measurements for eICIC/FeICIC are not restricted to subframes indicated by the pattern, unless the RSRP is used to derive an RSRQ measurement, which, however, is not the assumption for deriving relative RSRP requirements for eICIC/FeICIC.